1. Field of the Invention
The present invention relates to a power circuit of a vehicular electronic control unit incorporating a microprocessor for performing, for example, an ignition control and a fuel injection control on a vehicle engine. In particular, the invention relates to improvement of a power circuit for supplying control power to a vehicular electronic control unit exceptionally in spite of a power switch's being in an off-state.
2. Description of the Related Art
A technique of using both of a sleep power circuit and a control power circuit as power circuits of a vehicular electronic control unit are widely put in practical use. The sleep power circuit directly connects a vehicle battery to the vehicular electronic control unit to maintain data stored in a RAM that cooperates with a microprocessor that is incorporated in the vehicular electronic control unit. The control power circuit connects the vehicle battery to the vehicular electronic control unit at the output contact of an electromagnetic relay having a coil that is energized by the vehicle battery via a power switch.
Once activated, the electromagnetic relay is supplied with an operation maintenance signal from the microprocessor and continues its relaying state until the microprocessor completes various save controls even if the power switch is turned off. This kind of power supply state is called sleep power supply.
As exemplified above, many methods for continuing supply of power for a prescribed time after turning-off of a power switch have been proposed. For example, Japanese patent application JP-A-5-18315 (assigned to the same assignee of this application) discloses a concept that power shutoff of a vehicular electronic control unit (ECU) is delayed to allow recovery, to an initial position, of a stepping motor that is one of vehicle electric loads. The delay of power shutoff secures a time to transfer part of learned information stored in a RAM to a nonvolatile memory such as an EEPROM and make it stored there.
Another prior art technique for supplying control power to a vehicular electronic control unit though a power switch is in an off-state is disclosed in Japanese patent application JP-A-2003-74421 (assigned to DENSO CORP). That is, JP-A-2003-74421 discloses what is called a regular activation concept that a vehicular computer is supplied with power every predetermined time after a stop of the engine, whereby the states of particular sensors such as a fuel tank internal pressure sensor, a remaining fuel sensor, and a tank internal temperature sensor are monitored and occurrence/non-occurrence of fuel leakage or transpiration is judged.
In the techniques of JP-A-5-18315 and patent JP-A-2003-74421, the delayed power shutoff or the regular activation is performed individually, that is, the power circuit is not configured so that both of the delayed power shutoff and the regular activation can be performed in parallel. The technique of JP-A-5-18315 has a problem that if the microprocessor is activated regularly while the power switch is off, unnecessary power supply to various vehicle electric loads occurs to increase the consumption of current supplied from the vehicle battery.
In the power circuit of JP-A-2003-74421, vehicle electric loads are supplied with power directly from the power switch. Therefore, the power switch needs to deal with a large current and the delayed power shutoff cannot be performed.